Field of the Invention
The present invention concerns a method to implement a multi-echo magnetic resonance (MR) measurement sequence and an MR system to execute such a method. In particular the invention concerns techniques that enable an increased spatial resolution given predetermined time difference between successive gradient echoes, in which a predetermined maximum gradient pulse amplitude and a predetermined maximum gradient pulse rate-of-change are not exceeded.
Description of the Prior Art
Multi-echo measurement sequences of magnetic resonance (MR) imaging are known in which multiple MR images with different echo times are respectively acquired from different anatomical slices of an examined person. Due to the different echo times, the multiple MR images typically have different contrasts. The MR images with different contrasts can be used in what are known as chemical shift techniques in which a separation of different spin species occurs.
Multi-echo measurement sequences are frequently implemented such that MR images are obtained at very specific and well-defined echo times. For example, the concrete selection of the echo times can depend on the desired application of the MR images. One example of a typical application would be fat/water separation. The sought echo times are typically dependent on the strength of the basic magnetic field (field strength-dependent). The echo time (TE1) of a first MR image and the time interval or the time difference between the echo times of successively acquired MR images (δTE) decreases in an inverse proportion to the strength of the basic magnetic field of the MR system. Typical basic magnetic field strengths are 1.5 Tesla, 3 Tesla, 5 Tesla or 7 Tesla, for example.
Various types of multi-echo measurement sequences are known. In a conventional multi-echo measurement sequence, all detected MR echoes are detected (meaning at the various echo times) respectively as a time period after a radiation of radio-frequency (RF) pulse for excitation of the transverse magnetization of nuclear spins of a subject (RF excitation pulse). In other words: a number n of MR echoes is respectively detected in each of separated repetition intervals (TR intervals) after an RF excitation pulse. Therefore, such techniques are also known to those skilled in the art as an n-echo n-TR approach. N-echo n-TR techniques are known in connection with the detection of gradient echoes, for example.
The resolution of an MR image in the readout direction (frequency coding direction) is typically defined by the Fourier pixel size Δx. The Fourier pixel size is the size of a field of view in the readout direction, divided by the number of readout points Nx. The field of view designates a region of an examination subject that is depicted by the MR image. The smaller the Fourier pixel size Δx, the higher the resolution. The Fourier pixel size is inversely proportional to the 0th moment of the readout gradient:Δx=2π/(γM0x).γ is the gyromagnetic ratio. For water protons, the gyromagnetic ratio is γ/(2π)=42.576 MHz/T. The 0th moment of the readout gradient is the time integral of the amplitude of the readout gradient during the readout time, frequently also designated as an “area” of the readout gradient. If the readout gradient is thus constant during the entire readout time, the 0th moment M0x is then the product of amplitude of the readout gradient and readout time.
In gradient echo imaging, a switch is frequently made between the excitation and readout gradients of a pre-phasing gradient pulse in the readout direction whose 0th moment has the same magnitude as the moment of the readout gradient between the beginning of the readout gradient and the echo point in time. The direction of the pre-phasing gradient pulse is typically opposite the direction of the readout gradient, such that the total moment disappears exactly at the echo point in time. The echo time is often the time between the center of the excitation pulse and the echo point in time. For example, the echo time can be the time between a spin echo and the echo point in time.
Since the maximum amplitude of a gradient pulse and the shortest rise time can typically be technologically and/or physiologically limited, the maximum resolution with gradient echo-based n-echo n-TR techniques is thus conventionally limited by the shortest required gradient echo time TE1, but is not additionally limited by the shortest time difference ΔTE of successive gradient echoes. However, the total duration that is required to implement the multi-echo MR measurement sequence (measurement duration) is comparably long. Moreover, such a technique frequently extends the time interval between the detection of the different gradient echoes. This can lead to negative effects, particularly in measurements that are implemented to avoid breathing artifacts while an examined person holds his breath. Moreover, time-dependent drifts of the basic magnetic field, such as due to physiological processes or heating during the measurement—can lead to additional phase differences between the individual MR images with different echo points in time. A subsequent evaluation of the MR images then can be possible only to a limited extent, and possible quantitative analyses can be plagued with a relatively large error.
Multi-echo measurement sequences are known other than the n-echo n-TR-based measurement sequence described above. For example, multi-echo measurement sequences are also known that detect multiple echoes at different echo points in time or echo times after a single RF excitation pulse. The detection of multiple echoes following one RF pulse is also called an n-echo per TR technique. n-echo per TR techniques have the advantage of a reduced measurement duration. Due to the predetermined different echo points in time, given such multi-echo measurement sequences a maximum achievable spatial resolution is typically limited by the first echo time TE1, and additionally by the time difference between successive echoes. It is of particular significance that the time period provided for the detection of an echo is also limited, because the next echo should already be formed and detected after the time period ΔTE.
The maximum gradient amplitude and/or a maximum rise time and fall time of gradient fields or, respectively, rate-of-change of an MR system is often technologically and/or physiologically limited. For example, for the detection of gradient echoes it is often necessary to initially switch pre-phasing gradient pulses and to subsequently switch readout gradient fields during the readout of the gradient echo. Since the time period available for this is typically limited by the predetermined different echo points in time or the time difference between successive echo points in time, the maximum 0th moment M0x of the readout gradients (and therefore the achievable spatial resolution) is often limited accordingly.